Hello and thanks for researching this very important question!
First off, I have to mention the possibility of pregnancy in an animal that may have been misidentified as male. Gerbils are the most subtle of the rodent species in this regard, so you might want to examine his underside for the presence of nipples. What does he do if you put in some lengths of toilet paper?Gender assignment diagrams here
It is unusual for animals that eat fibrous plants and oily seeds to become constipated without some other health condition at play. Oils are very hard on the digestive organs, so natural plant matter such as cooked oatmeal or baby food squash are best and safest.
Some animals come home from pet stores incubating viral illnesses that do not show up for some time. Or they might be carrying bacterial or fungal agents that can cause problems.
If you suspect he is dehydrated, then a consultation with a veterinarian who knows what to do and how to do it safely is best:Exotic Mammal Veterinarian directory here
Fluids can be given under the skin to improve hydration, preserve the kidneys and get some water into his bowels. An antibiotic may be prescribed if the vet suspects an infective illness.
There is now a well-established association between raisins, grapes and kidney failure in dogs. For this reason, I do not give any of my pets grapes or raisin. I use dried cranberries as a Vitamin C source for my rodent colonies.
Animals living in the wild can well run off the excess calories in large oily seeds and fruit, so I would suggest the following diet recipe:
1) 1 3-pound bag of traditional Hamster/Gerbil seed mix.
2) 1 box original style Grape Nuts cereal (by Post).
3) 1 pouch of dried cranberries
4) 2 cups mixed seeds (Brown's)
The pressed hay blocks are full of sugar, and may even be rancid, so stick with loose Timothy Hay.
If mealworms gross you out, or are inconvenient to get, boil an egg, cool to room temperature, put it whole (with shell on) in a plastic bag and smash it with a rolling pin. Feed a pinch for each animal daily.
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